(1) Field of the Invention
The present invention relates to an uneven-surface detection apparatus for detecting fingerprints and the like in a personal identification apparatus.
(2) Description of the Related Art
In the high-tech information world of today, a strong demand has recently arisen for better computer system security techniques. In particular, in order to protect data confidentially a reliable identification must be made of personnel in charge of such systems, and thus strict checks on entrance to a computer room have become very important. In the past, passwords and ID cards have been used and now personal identification systems using fingerprints are in increasing use.
In a first conventional method for inputting uneven-surface data (e.g., fingerprint), a fingerprint is printed on paper using ink and is then sensed using an image sensor. In a second method, a light beam is incident on a glass/air interface at a critical angle or larger, and an uneven-surface pattern is thus instantaneously obtained.
In the first method, a finger must be stained with ink each time the fingerprint data is to be input, and an uneven coating or blurring of the ink makes the input operation difficult.
In the second method an uneven-surface data detection apparatus uses a prism, a finger surface (uneven-surface pattern having projections (convexes) and recesses (concaves)) is pressed against an oblique side portion of the prism, and illuminated light is incident on the oblique side portion thereof at a critical angle or larger. The incident light is scattered by projections and is totally reflected by the glass/air interface in recesses. The light is then incident on a detector (e.g., an imaging element), thereby allowing detection of the uneven-surface pattern.
However, light leaks through the unclean oblique surface due to remaining fingerprints or moisture. This leakage light irradiates the concave portion of the fingerprint and is scattered. Therefore, the level of signal light from the convex portion is lowered, degrading the contrast of the uneven-surface pattern. Since a prism is used, the total thickness of the apparatus cannot be reduced. More specifically, if the uneven-surface pattern of the entire palm is to be detected, a large prism must be used, resulting in a bulky apparatus. In addition, the resultant image may be distorted to a trapezoid shape due to a differenced in the optical path length from the surface of the palm to the detector.